The immune system is composed of many interdependent cell types that collectively protect the body from bacterial, parasitic, fungal, viral infections and from the growth of tumour cells. The guards of the immune system are macrophages that continually roam the bloodstream of their host. When challenged by infection or immunisation, macrophages respond by engulfing invaders marked with foreign molecules known as antigens. This event, mediated by helper T cells, sets forth a complicated chain of responses that result in the stimulation of B-cells. These B-cells, in turn, produce proteins called antibodies, which bind to the foreign invader. The binding event between antibody and antigen marks the foreign invader for destruction via phagocytosis or activation of the complement system. Five different classes of antibodies, or immunoglobulins, exist: IgA, IgD, IgE, IgG, and IgM. They differ not only in their physiological roles but also in their structures. From a structural point of view, IgG antibodies are a particular class of immunoglobulins that have been extensively studied, perhaps because of the dominant role they play in a mature immune response.
The biological activity, which the immunoglobulins possess, is today exploited in a range of different applications in the human and veterinary diagnostic, health care and therapeutic sector. In fact, in the last few years, monoclonal antibodies and recombinant immunoglobulin constructs have become the largest class of proteins currently investigated in clinical trials and receiving FDA approval as therapeutics and diagnostics. Complementary to expression systems and production strategies, purification protocols are designed to obtain highly pure antibodies in a simple and cost-efficient manner.
Traditional methods for isolation of immunoglobulins are based on selective reversible precipitation of the protein fraction comprising the immunoglobulins while leaving other groups of proteins in solution. Typical precipitation agents being ethanol, polyethylene glycol, lyotropic i.e. anti-chaotropic salts such as ammonium sulphate and potassium phosphate, and caprylic acid. Typically, these precipitation methods are giving very impure products while at the same time being time consuming and laborious. Furthermore, the addition of the precipitating agent to the raw material makes it difficult to use the supernatant for other purposes and creates a disposal problem, which is particularly relevant when speaking of large-scale purification of immunoglobulins.
Ion exchange chromatography is another well-known method of protein fractionation frequently used for isolation of immunoglobulins. However, since the charged ion exchange ligands will react with all oppositely charged compounds, the selectivity of ion exchange chromatography may be somewhat lower than other chromatographic separations.
Protein A and Protein G affinity chromatography are popular and widespread methods for isolation and purification of immunoglobulins, particularly for isolation of monoclonal antibodies, mainly due to the ease of use and the high purity obtained. Used in combination with ion exchange, hydrophobic interaction, hydroxyapatite and/or gel filtration steps, especially protein A-based methods have become the immunoglobulin purification method of choice for many biopharmaceutical companies. However, despite their common usage, there is a growing need and demand for effective alternatives addressing familiar problems associated with protein A-based media, such as cost, leakage and instability at increased pH values.
Hydrophobic interaction chromatography (HIC) is also a method widely described for isolation of immunoglobulins. However, hydrophobic matrices require an addition of lyotropic salts to the raw material to make the immunoglobulin bind efficiently. The bound immunoglobulin is released from the matrix by lowering the concentration of lyotropic salt in a continuous or stepwise gradient. If a highly pure product is the object, it is recommended to combine the hydrophobic chromatography with a further step. Thus, a disadvantage of this procedure is the necessity to add lyotropic salt to the raw material as this gives a d problem and thereby increased cost to the large-scale user. For other raw materials than cell culture supernatants such as whey, plasma, and egg yolk the addition of lyotropic salts to the raw materials would in many instances be prohibitive in large-scale applications as the salt could prevent any economically feasible use of the immunoglobulin depleted raw material. An additional problem in large-scale applications would be the disposal of several thousand liters of waste.
Thiophilic adsorption chromatography was introduced by J. Porath in 1985 (J. Porath et al; FEBS Letters, vol. 185, p. 306, 1985) as a new chromatographic adsorption principle for isolation of immunoglobulins. In this paper, it is described how divinyl sulphone activated agarose coupled with various ligands comprising a free mercapto-group show specific binding of immunoglobulins in the presence of 0.5 M potassium sulphate, i.e. a lyotropic salt. It was postulated that the sulphone group, from the vinyl sulphone spacer, and the resulting thioether in the ligand was a structural necessity to obtain the described specificity and capacity for binding of antibodies. It was however later shown that the thioether could be replaced by nitrogen or oxygen if the ligand further comprised an aromatic radical (K. L. Knudsen et al, Analytical Biochemistry, vol. 201, p. 170, 1992). Although the matrices described for thiophilic chromatography generally show good performance, they also have a major disadvantage in that it is needed to add lyotropic salts to the raw material to ensure efficient binding of the immunoglobulin, which is a problem for the reasons discussed above.
Other thiophilic ligands coupled to epoxy activated agarose have been disclosed in (J. Porath et. al. Makromol. Chem., Makromol. Symp., vol. 17, p. 359, 1988) and (A. Schwarz et. al., Journal of Chromatography B, vol. 664, pp. 83-88, 1995), e.g. 2-mercaptopyridine, 2-mercaptopyrimidine, and 2-mercaptothiazoline. However, all these affinity matrices still have inadequate affinity constants to ensure an efficient binding of the immunoglobulin without added lyotropic salts.
Liu et al (Yang Liu, Rui Zhao, Dihua Shangguan, Hongwu Zhang, Guoquan Liu: Novel sulfinethazine ligand used for one-step purification of immunoglobulin G from human plasma, Journal of Chromatography B, 792 (2003) 177-185) investigated the affinity of sulfinethazin (SMZ) to human IgG. Thus, a ligand is disclosed, which comprises a sulphonyl group wherein the R group is a heterocyclic ring. According to this article, SMZ was immobilised on monodisperse, non-porous, cross-linked poly(glycidyl methacrylate) beads. The beads were then used in high-performance affinity chromatography for isolation of IgG from human plasma. Maximal adsorption was achieved at pH 5.5. The beads presented minimal non-specific interaction with other proteins. Thus, the ligands were capable of adsorbing antibodies, while their interaction with other proteins was just sufficient to provide retardation thereof in the adsorption buffer used. However, as is well known, ester compounds such as methacrylate are easily hydrolysed at increased pH values. Consequently, similar to Protein A and Protein G matrices, the therein disclosed separation matrix would be expected to unstable at the commonly used cleaning in place (cip) procedures.
U.S. Pat. No. 4,725,355 relates to a body fluid purification medium comprising a support and an adsorbent, which includes at least one sulfa drug, for adsorbing and removing a pathogenic substance in a body fluid. The sulfa drug is a chemotherapeutic agent, and more specifically a sulfonamide characterised by aromatic R group(s). The medium can be provided in a body fluid flow path provided in a container between body fluid inlet and outlet ports.
However, there is still a need of alternative methods for purification of antibodies or immunoglobulin, which observe the demands of purity, safety, potency and cost effectiveness.